1. Field of the Invention
The invention relates to a position detecting device for finding the position of a specimen arranged in a conductive structure on the basis of microwaves, comprising an electrical circuit for producing and/or receiving the microwaves and a microwave antenna arrangement for emitting and/or receiving the microwaves.
2. Description of the Related Art
Such a position detecting device is for example described in the German patent publication DE 102 05 904 A1. The conductive structure is constituted by a housing of a pneumatic cylinder or of a hydraulic cylinder, in which a piston is arranged for longitudinal motion. The position detecting device serves for finding the distance of the piston with respect to a longitudinal terminal position, in the vicinity of which the microwave antenna arrangement is arranged. The microwave antenna arrangement comprises a coupling probe, which is held by a dielectric holding system. The housing or, respectively, the conductive structure cooperates with the coupling probe as regards the propagation of the microwaves. It is hence essential for the distance of the coupling probe from the housing be constant, that is to say that the dielectric holding system ensures having this constant distance.
The accuracy of measurement of the known position detecting device depends on the degree of precision with which the coupling probe is held by the dielectric holding system, on the distances of the dielectric holding system in relation to the conductive structure and, respectively, the housing of the cylinder and furthermore on the electrical conductive connections between the coupling probe and the electrical circuit for producing and respectively receiving the microwaves and the conductors between such circuit and the conductive structure and the cylinder housing respectively. Accordingly the requirements set for the accuracy of manufacture are quite substantial if a sufficient accuracy of measurement is aimed at. Owing to the mechanical loads, for example as a result of pressure, vibrations and the like, furthermore the distances between the individual components of the microwave antenna arrangement and/or the electrical circuit may vary so that signal quality of the microwaves and ultimately of the position-related measurement signal become worse even to the stage of complete failure of the position detecting device.